SECTION XI.
LYCOPODIACEÆ, OR CLUB MOSSES.
The Club Mosses are mostly perennial plants, with slender creeping stems, often several feet or yards in length, occasionally erect, and clothed by small, sessile, closely set, often imbricated leaves without veins. They have in some instances a habit resembling that of Conifers. The stems consist of a mass of thick-walled, often dotted cells, enclosing one or many bunches of scalariform tissue, which sends off branches to every leaf and bud. The scalariform tissue is accompanied by fine, elongated, and sometimes rather coarser cells, which are occasionally reticulated. The stem approaches to that of ferns, but the bundles of vascular tissue are confined to the centre. The branches of the stems are often bifurcate, and terminate in one or a pair of cone-like spikes, which are either sessile or stalked. The sporangia are sessile in the axils of the imbricated leaves or bracts that cover the cones. Two kinds grow on the same plant, one of them bivalved, containing a powdery substance, whose particles a high magnifying power shows to be globular spermatozoids: these do not germinate; and the other three-valved, enclosing comparatively large, nearly spherical granules, marked with three prominent ridges, radiating from one extremity. These certainly germinate, forming by cell-division a prothallus of hexagonal cellular tissue, adherent to and confluent with the spores, as in Marsileaceæ, or penetrating their cavity, but without the protrusion of threads, as in ferns or mosses. On this prothallus, archegonia are soon produced, the embryo being formed from a cell at its base; this sends down roots on one side, and a minute stem with the two primary leaves of the young plant at the other. The plant thus bears a close resemblance to a young plant of the dicotyledons, and exhibits, with regard to fructification, the highest organization of which the Cryptogams are capable, although in their tissues the Lycopods are inferior to the Equisetaceæ and Marsileaceæ.
There are four genera of this order, very widely dispersed, most abundant and larger in the hot moist parts of India and the Indian islands, but large tracts are covered with them within the arctic circles and in temperate latitudes. There are 200 species of the genus Lycopodium, or Wolfs-claw, and of these, six only are indigenous in Britain; they grow in very exposed situations, as in the case of L. clavatum, which lives on upland heaths and pastures, and which has a procumbent stem, creeping for many feet or yards, sending out branches in all directions, with a pair of cones at their extremities, and strong roots at intervals to fix it to the ground. Lycopodium inundatum forms large patches on the marshes in the south of England. New Zealand has many more species of this genus than Britain, and some of the noblest specimens. The dried spores of the L. clavatum are so inflammable, that they have been used on the stage to produce the effect of lightning. Some Club Mosses yield a blue dye, a colour which is now obtained of a better quality from coal tar. Others possess cathartic properties, but although they have been used as medicine, they are very dangerous from the violence of their effects.
The genus Selaginella has a moss-like habit. The stem is generally creeping, and flat-looking from being clothed with distichous leaves; but sometimes it is tall and erect, having feathery branches clad with leaves. The sporangia are sacs, with two or three valves containing large spores and sub-globose antheridia, containing orange-coloured or minute scarlet bodies, ultimately developed into spermatozoids. The species are very numerous and greatly varied, and mostly tropical.
From a comparison of the structure of the stems of the fossil plants, Sigillaria and Lepidodendron, Dr. Hooker concludes that they are highly developed Lycopods, approaching closely in structure to the highest class of plants. The stems of the former differ chiefly in size from those of the Lycopodium, and the cones of the Lepidostrobus differ in their greater development and in the thickness of their scales from those of the Conifers. The spores of the Lepidostrobus ornatus too are sphericotetrahedral, like those of the Lycopodium. But inferior as the existing Lycopods are to their fossil ancestors, their analogy to the Conifers gives them a more exalted position in the vegetable world than their tall and graceful allies, ‘the tree ferns.’
The gradual change of structure from the lowest to the highest cryptogamic form is accompanied by a singular variety in the mode of reproduction, and a degree of vital energy scarcely to be expected, at least in beings of such low organization as the yeast plant, which produces gemmæ in vast and rapid profusion, each gem or bud being only a facsimile of the parent cell. In like manner the mother plant is reproduced by the germination of the green zoospores, the green globular cells in the fronds of the lichens, the motile gonidia, and the discs in the baskets of the Marchantia, especially those of the Marchantia polymorpha, which have such vigorous vegetation that they form stomates on whichever side is turned to the light, and roots on the other. The result of all these is an individual perfectly similar to its parent, like that produced by a bud and cutting of a tree, or the axillary fruit buds of the Begonia. The leaves of that flowering plant, as well as those of the Achimenes and Gloxinia, possess the property of reproducing the parent plant, for, when laid on moist earth and slit in different places, a young plant rises from the upper side of the fracture, and roots shoot down from the under. Although this manner of growth resembles the germination of the embryo of an archegonium, it is widely different, for the embryonic cell is fertilized by the spermatozoids, so there is a certain analogy but not the smallest affinity. The highest vegetable classes can reproduce the mother plant in many ways, but they have nothing akin to the alternation of generations exhibited by many of the lower tribes, nor yet to conjugation like the Desmidiaceæ and Diatoms. Possibly the spores resulting from these two modes of reproduction, as well as the resting spores, may produce new species; certainly those resulting from fructification do occasionally yield new varieties.
Many spores produce the plant directly, others indirectly, as most of the fungi; but if the definition of a perfect plant be that which bears the fructification, the mycelium of a mushroom constitutes the plant, for the mushroom itself is only a kind of sporangium or spore-case. Nevertheless, the spores of the Puccinia and other microscopic fungi, which are the cause of the rust and mildew in wheat, give rise to a kind of prothallus, ‘a slight fore-shadowing’ of the prothallus of the Marchantia and Mosses, which only produce sporangia, and those of the Ferns, Horsetails, and Club Mosses, whose archegonia contain the embryo of the plant itself. The fructification of the Lycopods is the highest of which the Cryptogamia are capable, and brings them into a singular analogy with the flowering class. For in the ovule, or seed-vessel of the flower-bearing race, a large cell is formed, containing mucilaginous matter, which, soon after fructification, is converted into a mass of cellular tissue, which gives rise to the cell containing the embryo, just as the prothallus of a Lycopod gives rise to the archegonium containing the embryo of the plant. The linear leaves of existing Lycopods and the cones of their fossil allies, are similar to the leaves and cones of the Coniferæ: there is no affinity, but an analogy existing between these two groups sufficient to make them form links between the two great divisions of the vegetable kingdom. The Lycopodiaceæ probably may be regarded as the highest of the Cryptogamia, and Coniferæ as the lowest of the flower-bearing class.